This invention relates generally to methods of polymerization and the polymers formed thereby. In particular, this invention relates to methods using pulsed microwave polymerization for the synthesis of polymers such as polyamides, polyesters, and polyamideesters.
Despite extensive research and development directed to methods for the manufacture of polymers, there are still areas where improvement would result in significant economic benefit. Examples include shortening the time required for polymerization, and maximizing the efficiency of the energy input required for polymerization. There is also a continuing need for improved methods for the manufacture of three-dimensional molded parts having fine-scale features, especially those having close tolerances.
One approach to reducing times and input energies required for polymerization reactions is to employ microwave radiation as a heat source. As an alternative to conventional heating techniques, microwave irradiation provides an effective, selective, and fast synthetic method by heating the molecules directly through the interaction between the microwave energy and the molecular dipole moments of the starting materials. Microwave radiation has been employed in polymer synthesis and curing; see, for example, F. Parodi, Chim. Ind. (Milan), volume 80, number 1, pages 55-61 (1998); and U.S. Pat. No. 5,317,081 to Gelorme et al. A publication by Albert et al. describes microwave-activated polymerization of epsilon-caprolactone with titanium tetrabutylate catalyst, but this process appears to offer no advantages in reaction times or product molecular weights compared to the corresponding thermal synthesis; see P. Albert, H. Warth, R. Muelhaupt, and R. Janda, Macromol. Chem. Phys., volume 197, number 5, pages 1633-1640 (1996). In Great Britain Patent No. 1,534,151, Dolden et al. describe a microwave process for synthesis of polyamides from amino acids, lactams, or mixtures of organic diacids and organic diamines. While the method of Dolden et al. appears to enable reduced reaction times, it does not allow precise control of reaction temperature, and does not produce high molecular weight polymers in high yield. There accordingly remains a need for an efficient and highly reproducible process for microwave synthesis of polyamides, polyesters, and polyamideesters.
A time-saving and reproducible polymer synthesis is provided by a method, comprising:
irradiating a reaction mixture with microwaves, wherein
the microwave radiation is characterized by
a varying frequency of about 2.4 to about 7 GHz;
a frequency sweep rate of about 0.1 to about 10 GHz/sec;
a forward power input of about 200 to about 10,000 Watts per total moles of reactant; and
the forward power input of the microwave radiation is adjusted to maintain a known temperature in the composition; and further wherein
the reaction mixture comprises aliphatic lactones having from 4 to 12 carbon atoms; aliphatic lactams having from 4 to 12 carbon atoms; amino acids having from 4 to 12 carbon atoms; omega-hydroxyacids having from 4 to 12 carbon atoms; mixtures of aliphatic dicarboxylic acids having from 4 to 12 carbon atoms and aliphatic diamines having from 2 to 12 carbon atoms; mixtures of aliphatic dicarboxylic acids having from 4 to 12 carbon atoms and aliphatic diols having from 2 to 12 carbon atoms; or mixtures of aliphatic lactones having from 4 to 12 carbon atoms and aliphatic lactams having from 4 to 12 carbon atoms.
Use of microwave frequency sweeping and temperature control via microwave forward power setting and on/off forward power control enables synthesis of polymer having higher molecular weights and lower dispersities compared to polymers prepared by previously known methods.
The method enables substantial time savings compared to conventional thermal polymerizations. The method also enables solvent-free polymerizations and is suitable for directly producing molded articles by polymerizing monomer mixtures in article-shaped molds.
A method for synthesizing polyamides, polyester, and polyamideesters comprises:
irradiating a reaction mixture with microwaves, wherein
the microwave radiation is characterized by
a varying frequency of about 2.4 to about 7 GHz;
a frequency sweep rate of about 0.1 to about 10 GHz/sec;
a forward power input of about 200 to about 10,000 Watts per total moles of reactant; and
the forward power input of the microwave radiation is adjusted to maintain a known temperature in the composition; and further wherein
the reaction mixture comprises aliphatic lactones having from 4 to 12 carbon atoms; aliphatic lactams having from 4 to 12 carbon atoms; amino acids having from 4 to 12 carbon atoms; omega-hydroxyacids having from 4 to 12 carbon atoms; mixtures of aliphatic dicarboxylic acids having from 4 to 12 carbon atoms and aliphatic diamines having from 2 to 12 carbon atoms; mixtures of aliphatic dicarboxylic acids having from 4 to 12 carbon atoms and aliphatic diols having from 2 to 12 carbon atoms; or mixtures of aliphatic lactones having from 4 to 12 carbon atoms and aliphatic lactams having from 4 to 12 carbon atoms.
The method is suitable for the synthesis of polyesters from lactones. Suitable lactones include aliphatic lactones having from 4 to 12 carbon atoms, preferably from 5 to 10 carbon atoms. Preferred lactones may be represented by the formula 
wherein n=3-11. A highly preferred lactone is epsilon-caprolactone (n=5).
The method is also suitable for the synthesis of polyesters from omega-hydroxycarboxylic acids having from 4 to 12 carbon atoms. Preferred omega-hydroxycarboxylic acids are represented by the formula 
wherein n=3-11. A highly preferred hydroxycarboxylic acid is epsilon-hydroxycaproic acid (n-5)
The method is suitable for the synthesis of polyesters from mixtures comprising aliphatic dicarboxylic acids having 4-12 carbon atoms and aliphatic diols having 2-12 carbon atoms. Preferred aliphatic dicarboxylic acids are represented by the formula 
wherein n=2-10. A highly preferred aliphatic dicarboxylic acid is adipic acid (n=4). Preferred aliphatic diols are represented by the formula
HOxe2x80x94(CH2)nxe2x80x94OH
wherein n=2-12. Highly preferred organic diols include ethylene glycol and 1,4-butanediol. It is preferred that the molar ratio of the dicarboxylic acid to the diol be about 0.8:1.2 to about 1.2:0.8, more preferably about 0.9:1.1 to about 1.1:0.9, yet more preferably about 0.98:1.02 to about 1.02:0.98.
The method is suitable for the synthesis of polyamides from organic lactams having from 4 to 12 carbon atoms. Preferred lactams are represented by the formula 
wherein n=3-11. A highly preferred lactam is epsilon-caprolactam (n=5).
The method is suitable for the synthesis of polyamides from amino acids having from 4 to 12 carbon atoms. Preferred amino acids are represented by the formula 
wherein n=3-11. A highly preferred amino acid is epsilon-aminocaproic acid (n=5).
The method is suitable for the synthesis of polyamides from aliphatic dicarboxylic acids having from 4 to 12 carbon atoms and aliphatic diamines having from 2 to 12 carbon atoms. Suitable and preferred aliphatic dicarboxylic acids are the same as those described above for the synthesis of polyesters. Preferred aliphatic diamines are represented by the formula
H2Nxe2x80x94(CH2)nxe2x80x94NH2
wherein n=2-12. A highly preferred aliphatic diamine is hexamethylenediamine (H2N(CH2)6NH2). It is preferred that the molar ratio of the dicarboxylic acid to the diamine be about 0.8:1.2 to about 1.2:0.8, more preferably about 0.9:1.1 to about 1.1:0.9, yet more preferably about 0.98:1.02 to about 1.02:0.98.
The method is also suitable for the synthesis of polyamideesters from aliphatic lactones having from 4 to 12 carbon atoms and aliphatic lactams having from 4 to 12 carbon atoms. The aliphatic lactones are the same as those described above for polyester synthesis, and the aliphatic lactams are the same as those described above for the synthesis of polyamides. The ratio of aliphatic lactone to aliphatic lactam may vary widely depending on the desired composition of the final copolymer, as well as the relative reactivity of the lactone and the lactam. One advantage of the present method over conventional thermal copolymerizations of lactams and lactones is that the ratio of ester to amide units in the final copolymer may closely approximate the initial molar ratio of lactone to lactam. The present method therefore reduces waste and simplifies isolation procedures. A presently preferred initial molar ratio of aliphatic lactam to aliphatic lactone is about 0.5:1 to about 4.0:1, more preferably about 1:1 to about 2:1.
The composition may further comprise a catalyst or an initiator. Generally, any known catalyst or initiator suitable for the corresponding thermal polymerization may be used. Alternatively, the polymerization may be conducted without a catalyst or initiator. For example, in the synthesis of polyamides from aliphatic dicarboxylic acids and aliphatic diamines, no catalyst is required.
For the synthesis of polyamides from lactams, suitable catalysts include water and the omega-amino acids corresponding to the ring-opened (hydrolyzed) lactam used in the synthesis. Other suitable catalysts include metallic aluminum alkylates (MA1(OR)3H; wherein M is an alkali metal or alkaline earth metal, and R is C1-C12 alkyl), sodium dihydrobis(2-methoxyethoxy)aluminate, lithium dihydrobis(tert-butoxy)aluminate, aluminum alkylates (Al(OR)2R; wherein R is C1-C12 alkyl), N-sodium caprolactam, magnesium chloride or bromide salt of epsilon-caprolactam (MgXC6H10NO, X=Br or Cl), dialkoxy aluminum hydride. Suitable initiators include isophthaloybiscaprolactam, N-acetalcaprolactam, isocyanate epsilon-caprolactam adducts, alcohols (ROH; wherein R is C1-C12 alkyl), diols (HOxe2x80x94Rxe2x80x94OH; wherein R is R is C1-C12 alkylene), omega-aminocaproic acids, and sodium methoxide.
For the synthesis of polyesters from lactones, suitable catalysts include tin(II) compounds, such as stannous octoate (tin(II)2-ethylhexoate, C.A.S. Registry No. 301-10-0); and metal alkoxides such as alkoxides of Al, Zn, Y, Ti, and rare earth metals, wherein each alkoxide group has from 1 to 12 carbon atoms. Suitable initiators include aliphatic diols having from 2 to 12 carbon atoms, such as 1,4-butanediol.
For the synthesis of polyamideesters from lactones and lactams, suitable catalysts include metal hydride compounds, such as a lithium aluminum hydride catalysts having the formula LiAl(H)x(R1)y, where x=1-4, y=0-3, x+y=4, and R1 is selected from the group consisting of C1-C12 alkyl and C1-C12 alkoxy; highly preferred catalysts include LiAl(H)(OR2)3, wherein R2 is selected from the group consisting of C1-C8 alkyl; an especially preferred catalyst is LiAl(H)(OC(CH3)3)3. Other suitable catalysts and initiators include those described above for the polymerization of poly(epsilon-caprolactam) and poly(epsilon-caprolactone).
While the reaction can be conducted in the presence of air, it is generally preferred to exclude oxygen from the composition, as polymer products may oxidize and darken at the polymerization temperature in the presence of oxygen. Any known means of excluding oxygen may be employed, including flushing the system with an inert gas such as nitrogen or argon. While suitable results are demonstrated below for polymerization at atmospheric pressure, there is no particular limitation on the pressure during polymerization.
A microwave furnace suitable for the irradiating the composition comprises a microwave source, microwave frequency range selector, a microwave frequency modulator to modulate the microwave frequency across the selected frequency range, microwave forward power controller to select the forward power setting, a thermocouple or other temperature measuring means, and a microwave forward power on/off controller to turn the forward power on and off in response to the temperature of the composition. Frequency modulation increases the uniformity of the power distribution throughout the furnace cavity, thereby heating the composition uniformly. Suitable microwave furnaces are described in, for example, U.S. Pat. Nos. 5,321,222 and 5,961,871 to Bible et al., U.S. Pat. No. 5,648,038 to Fathi et al., and U.S. Pat. No. 5,521,360 to Johnson et al. A presently preferred microwave furnace is commercially available from Lambda Technologies, Inc., as model no. LT 502 Xb.
The selection of the actual microwave frequency range will depend on the reactants, but will generally be about 2.4 to about 7 GHz. The microwave frequency sweep range is generally about 0.1 to about 10 GHz/sec, with about 0.2 to about 5 GHz/sec being preferred and about 0.5 to about 1 GHz/sec being more preferred. A suitable forward power input is about 200 to about 10,000 Watts per total moles of reactant, preferably about 300 to about 6,000 Watts per total moles of reactant, more preferably about 300 to about 1500 Watts per total moles of reactant. Selection of a forward power input will depend on the nature of the reactants. For example, in the synthesis of nylon-6,6 from adipic acid and 1,6-diaminohexane, a preferred forward power level is about 1200 to about 4000 Watts, more preferably about 2000 to about 3500 Watts. In the synthesis of poly(caprolactone) from epsilon-caprolactone, a preferred forward power is about 500 to about 2500 Watts, more preferably about 900 to about 1500 Watts. In the synthesis of nylon-6 from epsilon-caprolactam, a preferred forward power is about 500 to about 3000 Watts, more preferably about 900 to about 2800 Watts. In the synthesis of poly(caprolactone-co-caprolactam) from mixtures of epsilon-caprolactone and epsilon-caprolactam, a preferred forward power is about 250 to about 1500 Watts, more preferably about 400 to about 1200 Watts.
It is preferred that the reaction mixture be irradiated in a vessel transparent to microwave radiation in the frequency range employed. Vessels comprising poly(tetrafluoroethylene) are presently preferred.
The method may additionally comprise a purification step to remove residual reactant and low molecular weight polymer. Purification can be accomplished, for example, by dissolving the crude polymer product in a solvent to form a solution and precipitating the purified product by addition of an anti-solvent to the solution. Alternatively, the purification may be accomplished by a solvent extraction of the crude polymer. In yet another alternative, purification may be accomplished by chromatographic separation of the high molecular weight polymer by, for example, size exclusion chromatography. All of the above methods are effective to remove residual reactant and low molecular weight polymer from the crude polymer. Such purification methods are known to those of ordinary skill in the art and details of their use may be determined without undue experimentation.
The method may additionally comprise a drying step. When applied to a crude reaction product, the drying step may remove residual reactant. When applied to a material derived from a purification step, as the extraction and crystallization methods described above, the drying step may remove residual solvents. Drying can be accomplished using one or more of elevated temperatures (preferably below the melting point of the polymer), reduced pressures, and atmosphere exchange.
The method is suitable for direct molding of shaped articles by filling a mold with a reaction mixture and irradiating the reaction mixture to form the plastic part. In this embodiment, lactones and lactams are preferred reactants because their polymerization produces no side products. This approach greatly simplifies and speeds up the production of molded parts.